Post-yield Behavior of Human Cortical Bone is Transversely Isotropic in Compression

INTRODUCTION Bone quality is largely dependent on the toughness of the tissue since age-related degradation in toughness makes bone more susceptible to fracture [1]. The observed decrease in bone toughness with age is mainly due to significant decreases in the capacity of the tissue to dissipate energy during the post-yield deformation [2]. Thus, understanding the post-yield behavior of bone is the key to establish the criteria of bone quality. However, what is actually happening during the post-yield deformation of bone at micro/ultrastructural levels is still poorly understood. As an effort to address the issue, we have studied the post-yield behavior of human cortical bone loaded in the longitudinal directions, showing that the compressive post-yield behavior of bone is associated with continuous microdamage accumulation, large plastic deformation (up to 5% plastic strain), and an increased involvement of the collagen phase in load bearing of the tissue [3]. In the present study, we intend to investigate the orientation dependence of the compressive post-yield behavior of human cortical bone using the novel progressive loading protocol previously developed in our laboratory. The hypothesis of this study is that the post-yield properties of human cortical bone are transversely isotropic, but the underlying mechanism of its plastic deformation is independent of loading orientations. Our objective is to acquire the fundamental information for accurate modeling of bone failure in the future.